A flexible fueled vehicle (FFV) has a single fuel tank, fuel system, and engine. The vehicle is designed to run on unleaded gasoline and an alcohol fuel (usually ethanol) in any mixture. The engine and fuel system in a FFV must be adapted slightly to run on alcohol fuels because they are corrosive. A special sensor (called a flex fuel sensor) in the fuel line of the FFV analyzes the fuel mixture and control the fuel injection and timing to adjust for different fuel compositions. The FFV offers its owner an environmentally beneficial option whenever an alternative (i.e., blended) fuel is available.
The Powertrain Control Module (PCM) of a FFV is able to adjust engine fuel control and ignition timing to match the percentage of ethanol/methanol (alcohol) content in the fuel. Common names for these commercially available fuels are E-22 for methanol fuel and E-85, which stands for up to 85 percent ethanol. The flex fuel (FF) sensor measures the percentage of alcohol and sends a signal to the PCM. Since the energy density of alcohol is much lower than gasoline and the mass fraction burned rate is much faster than gasoline, the PCM adjusts fuel/air mixture and spark advance timing to ensure power output is close to that of a normally fueled engine.
For a conventional FF sensor, the output frequency increases as the percentage of ethanol/methanol in the fuel mixture increases. For example, a fuel blend that is 30 percent methanol has an FF sensor signal output frequency between 60 and 100 Hz, and a 60 percent methanol blend will have an FF sensor frequency between 90 and 130 Hz. The PCM uses this frequency input to calculate the correct air/fuel ratio and spark advance for the vehicle. Regular 87 octane or 88 octane gasoline will yield a 40-60 Hz FF sensor output frequency.
One disadvantage of the conventional FF sensors is that they are expensive and, thus, drive up the cost of manufacture of a FFV hundreds of dollars in comparison with a similar gasoline-fueled vehicle. Thus, it is desirable to find a cheaper way to sense the degree of alcohol/gasoline blend in fuel provided the engine of a FFV.